1. Field of the Invention
This invention relates to the active matrix display device in which data bus lines are formed on a first transparent insulating substrate such as a glass panel and scan bus lines are formed on a second transparent insulating substrate, eliminating a grade-crossing of both bus lines on the same panel, and the method for driving the same.
More specifically, this invention concerns the active matrix display device in which the earth bus lines on the second substrate are further eliminated resulting in enlarging a relative display area per pixel (picture element) and obtaining a higher yield of panel fabrication. And further, this invention relates to the driving method for obtaining a high quality display, reducing a cross-talk problem.
2. Description of the Prior Art
An active matrix display device, together with a dot matrix type (or so-called simple matrix type), is widely used in a display terminal of information processing apparatus. Liquid crystal is usually enclosed in such devices as the display medium.
In the active matrix display device, each pixel once addressed maintains an active state until the next scan, therefore this type has advantages of avoiding reduction of contrast and angle of visibility such as would happen ordinarily for the dot matrix type.
FIG. 1 shows an equivalent circuit of the active matrix display device of the prior art. A thin film transistor (abbreviated as TFT hereinafter) 31 is used as an active switch element. TFT 31 provides gate electrode 32, drain electrode 33, and source electrode 34. Source electrode 34 is connected to display electrode of liquid crystal display element 35. Both scan bus lines 36 and data bus lines 37 are formed on the second transparent insulating substrate and two bus lines 36 and 37 are perpendicular with each other.
Liquid crystal display element 35 consists of two display electrodes and liquid crystal medium held therebetween. One display electrode is formed on the second substrate and connected to source electrode 34 of TFT 31, and the common earth electrode formed on the first substrate is utilized as another display electrode. Gate electrode 32 is connected to scan bus line 36, and drain electrode 33 is connected to data bus line 37.
TFT 31, one display electrode of liquid crystal display element 35, scan bus line 36, and data bus line 37 are formed on the second substrate such as a glass panel, and common display electrode is formed on the first substrate, and two substrates are sealed together at their peripheral regions, having a small distance apart and enclosing the liquid crystal medium.
By applying address pulses sequentially having such as 30 to 60 .mu.s pulse width, to scan bus line 36 and applying data pulses to each data bus line 37 synchronized with the above address pulse, the information is thus transmitted to each liquid crystal display element 35 arrayed in a row and is maintained for subsequent one frame period.
Next, the above performance is explained in more details. When the scan bus line 36 is addressed, TFT 31, wherein gate electrode 32 is connected to said scan bus line, is turned on, and thus data voltage is impressed to liquid crystal display element 35 through data bus line 37 and TFT 31, and the electro-static capacitance of liquid crystal display element 35 is charged up. The above liquid crystal display element can maintain the information by the charged voltage after TFT 31 is turned off. When the scan bus line 36 is addressed next time, the liquid crystal display element 35 is newly charged up corresponding to the new information.
In the active matrix display device above described, scan bus line 36 and data bus line 37 are formed insulated from each other and perpendicular to each other on the same substrate. Therefore, degradation of insulation or a short between two bus lines at the crossing point is often observed. In this case, it results in a line fault of display extending along the bus line, and the yield in fabrication is liable to fall because of the crossed bus line structure.
A step is formed for the upper bus line at the crossing point, the step having a height of total thickness of the lower bus line and an insulating layer. As a result, the upper bus line is liable to faults such as an increased resistance or a breakage thereof.
There are other problems to be solved for the above structure. Because scan bus lines 36 and data bus lines 37 are formed on the same glass panel, an opening rate, which is defined as a ratio of each display electrode area to each pixel area, is reduced.
To solve the above problem, the improved design has been proposed, which is disclosed in French patent Publication No. 2,553,218 by A. Chevenas-Paule and J. Frederic-Clerc, or Japanese Patent Tokugansho 60-274011 by K. Oki, S. Kawai, etal. FIG. 2 shows the equivalent circuit of the proposed design. On a first glass panel, data bus line 37 is formed as a common display electrode of liquid crystal display element 35. On a second glass panel, TFT 31, scan bus line 36 and another display electrode are formed, wherein gate electrode 32 is connected to scan bus line 36, drain electrode 33 is connected to the display electrode of liquid crystal display element 35, and source electrode is connected to a common earth bus line. Liquid crystal medium is held between two glass panels.
Though the directions of scan bus line 36 and data bus line 37 are perpendicular to each other, they are formed on separate panels. Therefore there is no problem such as insulation between scan bus line 36 and data bus line 37 and the step problem at the crossing point of the upper bus line encountered in FIG. 1 type.
The improved design of FIG. 2 can reduce defects caused in bus line formation and rise the fabrication yield. Though this type of design eliminates the crossing points of bus lines on the same glass panel, however, it necessitates a formation of additional earth bus lines parallel with scan bus lines 36 on the second glass panel, each source electrode 34 of TFT 31 is connected to this earth bus line. Therefore, the opening rate can not be increased large enough.